In the case of a building to which work remains to be done of a kind which is customarily done to a building of the type in question after the erection of the building has been substantially completed, it shall be assumed for the purposes of this Schedule that the erection of the building has been or can reasonably be expected to be completed at the expiration of such period, beginning with the date of its completion apart from the work, as is reasonably required for carrying out the work.